Fin field effect transistor

ABSTRACT

A fin field effect transistor including a first insulation region and a second insulation region over a top surface of a substrate. The first insulation region includes tapered top surfaces, and the second insulation region includes tapered top surfaces. The fin field effect transistor further includes a fin extending above the top surface between the first insulation region and the second insulation region. The fin includes a first portion having a top surface below the tapered top surfaces of the first insulation region. The fin includes a second portion having a top surface above the tapered top surfaces of the first insulation region.

PRIORITY CLAIM

The present application is continuation of U.S. application Ser. No. 13/859,505, filed Apr. 9, 2013, which is a divisional of U.S. application Ser. No. 12/903,712, filed Oct. 13, 2010, now U.S. Pat. No. 8,440,517, which are incorporated herein by reference in their entireties.

RELATED APPLICATIONS

The present application is related to U.S. patent application Ser. No. 12/707,788, filed on Feb. 18, 2010, titled MEMORY POWER GATING CIRCUIT AND METHODS; Ser. No. 12/758,426, filed on Apr. 12, 2010, titled FINFETS AND METHODS FOR FORMING THE SAME; Ser. No. 12/731,325, filed on Mar. 25, 2010, titled ELECTRICAL FUSE AND RELATED APPLICATIONS; Ser. No. 12/724,556, filed on Mar. 16, 2010, titled ELECTRICAL ANTI-FUSE AND RELATED APPLICATIONS; Ser. No. 12/757,203, filed on Apr. 9, 2010, titled STI STRUCTURE AND METHOD OF FORMING BOTTOM VOID IN SAME; Ser. No. 12/797,839, filed on Jun. 10, 2010, titled FIN STRUCTURE FOR HIGH MOBILITY MULTIPLE-GATE TRANSISTOR; Ser. No. 12/831,842, filed on Jul. 7, 2010, titled METHOD FOR FORMING HIGH GERMANIUM CONCENTRATION SiGe STRESSOR; Ser. No. 12/761,686, filed on Apr. 16, 2010, titled FINFETS AND METHODS FOR FORMING THE SAME; Ser. No. 12/766,233, filed on Apr. 23, 2010, titled FIN FIELD EFFECT TRANSISTOR; Ser. No. 12/757,271, filed on Apr. 9, 2010, titled ACCUMULATION TYPE FINFET, CIRCUITS AND FABRICATION METHOD THEREOF; Ser. No. 12/694,846, filed on Jan. 27, 2010, titled INTEGRATED CIRCUITS AND METHODS FOR FORMING THE SAME; Ser. No. 12/638,958, filed on Dec. 14, 2009, titled METHOD OF CONTROLLING GATE THICKNESS IN FORMING FINFET DEVICES; Ser. No. 12/768,884, filed on Apr. 28, 2010, titled METHODS FOR DOPING FIN FIELD-EFFECT TRANSISTORS; Ser. No. 12/731,411, filed on Mar. 25, 2010, titled INTEGRATED CIRCUIT INCLUDING FINFETS AND METHODS FOR FORMING THE SAME; Ser. No. 12/775,006, filed on May 6, 2010, titled METHOD FOR FABRICATING A STRAINED STRUCTURE; Ser. No. 12/886,713, filed Sep. 21, 2010, titled METHOD OF FORMING INTEGRATED CIRCUITS; Ser. No. 12/941,509, filed Nov. 8, 2010, titled MECHANISMS FOR FORMING ULTRA SHALLOW JUNCTION; Ser. No. 12/900,626, filed Oct. 8, 2010, titled TRANSISTOR HAVING NOTCHED FIN STRUCTURE AND METHOD OF MAKING THE SAME; Ser. No. 12/903,712, filed Oct. 13, 2010, titled FINFET AND METHOD OF FABRICATING THE SAME; 61/412,846, filed Nov. 12, 2010, 61/394,418, filed Oct. 19, 2010, titled METHODS OF FORMING GATE DIELECTRIC MATERIAL and 61/405,858, filed Oct. 22, 2010, titled METHODS OF FORMING SEMICONDUCTOR DEVICES. All of the above applications are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The disclosure relates to integrated circuit fabrication, and more particularly to a fin field effect transistor.

BACKGROUND

As the semiconductor industry has progressed into nanometer technology process nodes in pursuit of higher device density, higher performance, and lower costs, challenges from both fabrication and design issues have resulted in the development of three-dimensional designs, such as a fin field effect transistor (FinFET). A typical FinFET is fabricated with a thin vertical “fin” (or fin structure) extending from a substrate formed by, for example, etching away a portion of a silicon layer of the substrate. The channel of the FinFET is formed in this vertical fin. A gate is provided over (e.g., wrapping) the fin. Having a gate on both sides of the channel allows gate control of the channel from both sides. In addition, strained materials in recessed source/drain (S/D) portions of the FinFET utilizing selectively grown silicon germanium (SiGe) may be used to enhance carrier mobility.

However, there are challenges to implementation of such features and processes in complementary metal-oxide-semiconductor (CMOS) fabrication. For example, non-uniform distribution of strained materials causes non-uniformity of strains applied to the channel region of the FinFET, thereby increasing the likelihood of device instability and/or device failure.

Accordingly, what are needed are an improved device and a method for fabricating a strained structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a flowchart illustrating a method of fabricating a FinFET according to various aspects of the present disclosure; and

FIGS. 2A-10C are perspective and cross-sectional views of a FinFET at various stages of fabrication according to various embodiments of the present disclosure.

DESCRIPTION

It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Referring to FIG. 1, illustrated is a flowchart of a method 100 of fabricating a fin field effect transistor (FinFET) according to various aspects of the present disclosure. The method 100 begins with step 102 in which a substrate is provided. The method 100 continues with step 104 in which a fin is formed in the substrate. The method 100 continues with step 106 in which a dielectric material is deposited over the substrate and top portion of the dielectric layer is removed to form first and second insulation regions, so that top surfaces of the first and second insulation regions are below a top surface of the fin. The method 100 continues with step 108 in which a gate stack is formed over a portion of the fin and over a portion of the first and second insulation regions. The method 100 continues with step 110 in which a portion of the fin not covered by the gate stack is recessed to form a recessed portion of the fin below the top surfaces of the first and second insulation regions. The method 100 continues with step 112 in which corners of the top surfaces of the first and second insulation regions not covered by the gate stack is etched to form tapered top surfaces of the first and second insulation regions. The method 100 continues with step 114 in which a strained material is selectively grown over the recessed portion of the fin and the tapered top surfaces of the first and second insulation regions.

As employed in the present disclosure, the FinFET 200 refers to any fin-based, multi-gate transistor. The FinFET 200 may be included in a microprocessor, memory cell, and/or other integrated circuit (IC). It is noted that the method of FIG. 1 does not produce a completed FinFET 200. A completed FinFET 200 may be fabricated using complementary metal-oxide-semiconductor (CMOS) technology processing. Accordingly, it is understood that additional processes may be provided before, during, and after the method 100 of FIG. 1, and that some other processes may only be briefly described herein. Also, FIGS. 1 through 10C are simplified for a better understanding of the inventive concepts of the present disclosure. For example, although the figures illustrate the FinFET 200, it is understood the IC may comprise a number of other devices comprising resistors, capacitors, inductors, fuses, etc.

Referring to FIGS. 2A-10C, illustrated are various perspective and cross-sectional views of the FinFET 200 at various stages of fabrication according to various embodiments of the present disclosure.

FIG. 2A is a perspective view of the FinFET 200 having a substrate 202 at one of various stages of fabrication according to an embodiment, and FIG. 2B is a cross-sectional view of a FinFET taken along the line a-a of FIG. 2A. In one embodiment, the substrate 202 comprises a crystalline silicon substrate (e.g., wafer). The substrate 202 may comprise various doped regions depending on design requirements (e.g., p-type substrate or n-type substrate). In some embodiments, the doped regions may be doped with p-type or n-type dopants. For example, the doped regions may be doped with p-type dopants, such as boron or BF2; n-type dopants, such as phosphorus or arsenic; and/or combinations thereof. The doped regions may be configured for an n-type FinFET, or alternatively configured for a p-type FinFET.

In some alternative embodiments, the substrate 202 may be made of some other suitable elemental semiconductor, such as diamond or germanium; a suitable compound semiconductor, such as gallium arsenide, silicon carbide, indium arsenide, or indium phosphide; or a suitable alloy semiconductor, such as silicon germanium carbide, gallium arsenic phosphide, or gallium indium phosphide. Further, the substrate 202 may include an epitaxial layer (epi-layer), may be strained for performance enhancement, and/or may include a silicon-on-insulator (SOI) structure.

The fins are formed by etching into the substrate 202. In one embodiment, a pad layer 204 a and a mask layer 204 b are formed on the semiconductor substrate 202. The pad layer 204 a may be a thin film comprising silicon oxide formed, for example, using a thermal oxidation process. The pad layer 204 a may act as an adhesion layer between the semiconductor substrate 202 and mask layer 204 b. The pad layer 204 a may also act as an etch stop layer for etching the mask layer 204 b. In at least one embodiment, the mask layer 204 b is formed of silicon nitride, for example, using low-pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD). The mask layer 204 b is used as a hard mask during subsequent photolithography processes. A photo-sensitive layer 206 is formed on the mask layer 204 b and is then patterned, forming openings 208 in the photo-sensitive layer 206.

FIG. 3A is a perspective view of the FinFET 200 at one of various stages of fabrication according to an embodiment, and FIG. 3B is a cross-sectional view of a FinFET taken along the line a-a of FIG. 3A. The mask layer 204 b and pad layer 204 a are etched through openings 208 to expose underlying semiconductor substrate 202. The exposed semiconductor substrate 202 is then etched to form trenches 210 with top surfaces 202 s of the semiconductor substrate 202. Portions of the semiconductor substrate 202 between trenches 210 form semiconductor fins 212. Trenches 210 may be strips (viewed from in the top of the FinFET 200) parallel to each other, and closely spaced with respect to each other. Trenches 210 each have a width W, a depth D, and are spaced apart from adjacent trenches by a spacing S. For example, the spacing S between trenches 210 may be smaller than about 30 nm. The photo-sensitive layer 206 is then removed. Next, a cleaning may be performed to remove a native oxide of the semiconductor substrate 202. The cleaning may be performed using diluted hydrofluoric (DHF) acid.

In some embodiments, depth D of the trenches 210 may range from about 2100 Å to about 2500 Å, while width W of the trenches 210 ranges from about 300 Å to about 1500 Å. In an exemplary embodiment, the aspect ratio (D/W) of the trenches 210 is greater than about 7.0. In some other embodiments, the aspect ratio may even be greater than about 8.0. In yet some embodiments, the aspect ratio is lower than about 7.0 or between 7.0 and 8.0. One skilled in the art will realize, however, that the dimensions and values recited throughout the descriptions are merely examples, and may be changed to suit different scales of integrated circuits.

Liner oxide (not shown) is then optionally formed in the trenches 210. In an embodiment, liner oxide may be a thermal oxide having a thickness ranging from about 20 Å to about 500 Å. In some embodiments, liner oxide may be formed using in-situ steam generation (ISSG) and the like. The formation of liner oxide rounds corners of the trenches 210, which reduces the electrical fields, and hence improves the performance of the resulting integrated circuit.

FIG. 4A is a perspective view of the FinFET 200 at one of various stages of fabrication according to an embodiment, and FIG. 4B is a cross-sectional view of the FinFET taken along the line a-a of FIG. 4A. Trenches 210 are filled with a dielectric material 214. The dielectric material 214 may include silicon oxide, and hence is also referred to as oxide 214 in the present disclosure. In some embodiments, other dielectric materials, such as silicon nitride, silicon oxynitride, fluoride-doped silicate glass (FSG), or a low-K dielectric material, may also be used. In an embodiment, the oxide 214 may be formed using a high-density-plasma (HDP) CVD process, using silane (SiH4) and oxygen (O2) as reacting precursors. In other embodiments, the oxide 214 may be formed using a sub-atmospheric CVD (SACVD) process or high aspect-ratio process (HARP), wherein process gases may comprise tetraethylorthosilicate (TEOS) and/or ozone (O3). In yet other embodiments, the oxide 214 may be formed using a spin-on-dielectric (SOD) process, such as hydrogen silsesquioxane (HSQ) or methyl silsesquioxane (MSQ).

FIGS. 4A and 4B depict the resulting structure after the deposition of the dielectric material 214. A chemical mechanical polish is then performed, followed by the removal of the mask layer 204 b and pad layer 204 a. The resulting structure is shown in FIGS. 5A and 5B. FIG. 5A is a perspective view of the FinFET 200 at one of various stages of fabrication according to an embodiment, and FIG. 5B is a cross-sectional view of the FinFET taken along the line a-a of FIG. 5A. The remaining portions of the oxide 214 in the trenches 210 are hereinafter referred to as insulation regions 216. In at least one embodiment the mask layer 204 b is formed of silicon nitride, the mask layer 204 b may be removed using a wet process using hot H3PO4, while pad layer 204 a may be removed using diluted HF acid, if formed of silicon oxide. In some alternative embodiments, the removal of the mask layer 204 b and pad layer 204 a may be performed after the recessing of the insulation regions 216, which recessing step is shown in FIGS. 6A and 6B.

The CMP process and the removal of the mask layer 204 b and pad layer 204 a produce the structure shown in FIGS. 5A/5B. As shown in FIGS. 6A and 6B, the insulation regions 216 are recessed by an etching step, resulting in recesses 218. In one embodiment, the etching step may be performed using a wet etching process, for example, by dipping the substrate 202 in hydrofluoric acid (HF). In another embodiment, the etching step may be performed using a dry etching process, for example, the dry etching process may be performed using CHF3 or BF3 as etching gases.

The remaining insulation regions 216 may comprise flat top surfaces 216 t. The remaining insulation regions 216 may comprise first isolation region 216 a and second isolation region 216 b. Further, the upper portions 222 of the semiconductor fins 212 protruding over the flat top surfaces 216 t of the remaining insulation regions 216 thus are used to form channel regions of the FinFETs 200. The upper portions 222 of the semiconductor fins 212 may comprise top surfaces 222 t and sidewalls 222 s. Height H of the upper portions 222 of the semiconductor fins 212 may range from 15 nm to about 50 nm. In some embodiments, the height H is greater than 50 nm or smaller than 15 nm. For simplicity, the upper portion 222 of the semiconductor fin 212 between the first and second insulation regions 216 a, 216 b is hereinafter referred to as channel fin 222 a to illustrate each upper portion of the semiconductor fin 212, wherein the flat top surfaces 216 t of the first and second insulation regions 216 a, 216 b are lower than the top surface 222 t of the semiconductor fin 212.

The process steps up to this point have provided the substrate 202 having the first insulation region 216 a and the second insulation region 216 b having respective top surfaces 216 t, and a fin 212 between the first and second insulation regions 216 a, 216 b, wherein the top surfaces 216 t of the first and second insulation regions are lower than a top surface 222 t of the fin 212.

FIG. 7A is a perspective view of the FinFET 200 at one of various stages of fabrication according to an embodiment, and FIG. 7B is a cross-sectional view of the FinFET taken along the line a-a of FIG. 7A. A gate stack 220 is formed over the substrate 202 over the top surface 222 t and sidewalls 222 s of a non-recessed portion of the channel fin 222 a and extending to the flat top surfaces 216 t of the first and second insulation regions 216 a, 216 b. In some embodiments, the gate stack 220 comprises a gate dielectric layer 220 a and a gate electrode layer 220 b over the gate dielectric layer 220 a.

In FIGS. 7A and 7B, the gate dielectric 220 a is formed to cover the top surface 222 t and sidewalls 222 s of the channel fin 222 a. In some embodiments, the gate dielectric layer 220 a may include silicon oxide, silicon nitride, silicon oxy-nitride, or high-k dielectrics. High-k dielectrics comprise metal oxides. Examples of metal oxides used for high-k dielectrics include oxides of Li, Be, Mg, Ca, Sr, Sc, Y, Zr, Hf, Al, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and/or mixtures thereof. In the present embodiment, the gate dielectric layer 220 a is a high-k dielectric layer with a thickness in the range of about 10 to 30 angstroms. The gate dielectric layer 220 a may be formed using a suitable process such as atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), thermal oxidation, UV-ozone oxidation, or combinations thereof. The gate dielectric layer 220 a may further comprise an interfacial layer (not shown) to reduce damage between the gate dielectric layer 220 a and channel fin 222 a. The interfacial layer may comprise silicon oxide.

The gate electrode layer 220 b is then formed on the gate dielectric layer 220 a. In at least one embodiment, the gate electrode layer 220 b covers the upper portion 222 of more than one semiconductor fin 212, so that the resulting FinFET 200 comprises more than one fin. In some alternative embodiments, each of the upper portions 222 of the semiconductor fins 212 may be used to form a separate FinFET 200. In some embodiments, the gate electrode layer 220 b may comprise a single layer or multilayer structure. In the present embodiment, the gate electrode layer 220 b may comprise poly-silicon. Further, the gate electrode layer 220 b may be doped poly-silicon with the uniform or non-uniform doping. In some alternative embodiments, the gate electrode layer 220 b may include a metal such as Al, Cu, W, Ti, Ta, TiN, TiAl, TiAlN, TaN, NiSi, CoSi, other conductive materials with a work function compatible with the substrate material, or combinations thereof. In the present embodiment, the gate electrode layer 220 b comprises a thickness in the range of about 30 nm to about 60 nm. The gate electrode layer 220 b may be formed using a suitable process such as ALD, CVD, PVD, plating, or combinations thereof.

Still referring to FIG. 7A, the FinFET 200 further comprises a dielectric layer 224 formed over the substrate 202 and along the side of the gate stack 220. In some embodiments, the dielectric layer 224 may include silicon oxide, silicon nitride, silicon oxy-nitride, or other suitable material. The dielectric layer 224 may comprise a single layer or multilayer structure. A blanket layer of the dielectric layer 224 may be formed by CVD, PVD, ALD, or other suitable technique. Then, an anisotropic etching is performed on the dielectric layer 224 to form a pair of spacers 224 on two sides of the gate stack 220. The dielectric layer 224 comprises a thickness ranging from about 5 to 15 nm.

FIG. 8A is a perspective view of the FinFET 200 at one of various stages of fabrication according to an embodiment, and FIG. 8B is a cross-sectional view of the FinFET taken along the line b-b of FIG. 8A. The portion of the semiconductor fin 212 not covered by the gate stack 220 and spacers 224 formed thereover are recessed to form a recessed portion 226 of the fin 212 having a top surface 212 r below the flat top surfaces 216 t of the first and second insulation regions 216 a, 216 b. In one embodiment, using the pair of spacers 224 as hard masks, a biased etching process is performed to recess top surface 222 t of the channel fin 222 a that are unprotected or exposed to form the recessed portion 226 of the semiconductor fin 212. In an embodiment, the etching process may be performed under a pressure of about 1 mTorr to 1000 mTorr, a power of about 50 W to 1000 W, a bias voltage of about 20 V to 500 V, at a temperature of about 40° C. to 60° C., using a HBr and/or Cl2 as etch gases. Also, in the embodiments provided, the bias voltage used in the etching process may be tuned to allow better control of an etching direction to achieve desired profiles for the recessed portion 226 of the semiconductor fin 212.

FIG. 9A is a perspective view of the FinFET 200 at one of various stages of fabrication according to an embodiment, and FIG. 9B is a cross-sectional view of the FinFET taken along the line b-b of FIG. 9A. Subsequent to the formation of the recessed portion 226 of the semiconductor fin 212, corners of the flat top surfaces 216 t of the first and second insulation regions 216 a, 216 b not covered by the gate stack 220 are etched to form tapered top surfaces 216 u of the first and second insulation regions 216 a, 216 b. In one embodiment, the etching step may be performed using a wet etching process, for example, by dipping the substrate 202 in hydrofluoric acid (HF). In another embodiment, the etching step may be performed using a non-biased dry etching process, for example, the dry etching process may be performed using CHF3 or BF3 as etching gases.

In one embodiment, the tapered top surfaces 216 u of the first and second insulation regions 216 a, 216 b comprise a flat portion and sloped or beveled sidewalls (shown in FIGS. 9A and 9B). Therefore, a width W2 of the flat portion of the tapered top surface 216 u is less than a maximum width W1 of the flat top surface 216 u. In one embodiment, a ratio of the width W2 of the flat portion to a maximum width W3 of the first insulation region 216 a is from 0.05 to 0.95. Further, a distance D1 of a lowest point of the tapered top surface 216 u and the top surface 202 s of the substrate 202 is in the range of about 100 to 200 nm.

FIG. 9C is a cross-sectional view of another FinFET 200 embodiment. In the embodiment depicted in FIG. 9C, the corners of the flat top surfaces 216 t of the first and second insulation regions 216 a, 216 b not covered by the gate stack 220 are further removed until the flat portion of the tapered top surface 216 u disappear to form a curved top portion of the tapered top surface 216 u (shown in FIG. 9C). It is observed that the space between the neighboring semiconductor fins 212 have a middle line 228, and the curved top portion of the tapered top surface 216 u close to the middle line 228 is higher than the curved top portion of the tapered top surface 216 u close to the semiconductor fins 222. In other words, the tapered top surfaces 216 u comprise a highest point P in the middle of the tapered top surfaces 216 u. Further, a distance D2 of a lowest point of the tapered top surface 216 u and the top surface 202 s of the substrate 202 is in the range of about 100 to 200 nm. In one embodiment, the flat top surface 216 t is coplanar with the highest point P of the tapered top surface 216 u. In another embodiment, the flat top surface 216 t is higher than the highest point P of the tapered top surface 216 u. A distance D3 between the flat top surface 216 t and the highest point P of the tapered top surface 216 u is in the range of about 0.1 to 0.3 nm. In still another embodiment, the semiconductor fin 212 further comprises a non-recessed portion under a gate stack 220 having a top surface 222 t higher than the tapered top surfaces 216 u. A distance D4 between the top surface 222 t of the non-recessed portion of the semiconductor fin 212 and the highest point P of the tapered top surface 216 u is in the range of about 100 to 200 nm.

FIG. 10A is a perspective view of the FinFET 200 at one of various stages of fabrication according to an embodiment, and FIG. 10B is a cross-sectional view of the FinFET taken along the line b-b of FIG. 10A. FIG. 10C is a cross-sectional view of another FinFET 200 embodiment having strained material 230 formed over the structure depicted in FIG. 9C. Then, the structures depicted in FIGS. 10A, 10B, and 10C are produced by selectively growing a strained material 230 over the recessed portion 226 of the semiconductor fin 212 and extending over the tapered top surfaces 216 u of the first and second insulation regions 216 a, 216 b. Since the lattice constant of the strained material 230 is different from the substrate 202, the channel region of the semiconductor fin 212 is strained or stressed to enable carrier mobility of the device and enhance the device performance. In at least one embodiment, the strained material 230, such as silicon carbon (SiC), is epi-grown by a LPCVD process to form the source and drain regions of the n-type FinFET. The LPCVD process is performed at a temperature of about 400 to 800° C. and under a pressure of about 1 to 200 Torr, using Si3H8 and SiH3CH as reaction gases. In at least another embodiment, the strained material 230, such as silicon germanium (SiGe), is epi-grown by a LPCVD process to form the source and drain regions of the p-type FinFET. The LPCVD process is performed at a temperature of about 400 to 800° C. and under a pressure of about 1 to 200 Torr, using SiH4 and GeH4 as reaction gases.

In the present embodiment, the selective growth of the strained material 230 continues until the material 230 extends vertically a distance ranging from about 10 to 100 nm above the surface 202 a of the substrate 202 and extends laterally over the tapered top surfaces 216 u of the first and second insulation regions 216 a, 216 b. It should be noted that tapered top surfaces 216 u of the first and second insulation regions 216 a, 216 b make it easier for growth precursors to reach the growth surface during selective growth of the strained material 230 from different recessed portions 226 of the semiconductor fins 212 to eliminate voids under the merged strained materials 230. In some embodiments, the voids under the merged strained materials 230 reduce strain efficiency of the strained materials 230, i.e., the strained materials 230 with voids provide less strain into channel region of the FinFET than the configuration having no void formed in the strained materials 230, thereby increasing the likelihood of device instability and/or device failure. In the present embodiment, the strained material 230 has a substantially flat surface as the strained materials 230 grown from different recessed portions 226 are merged. Accordingly, the present method of fabricating a FinFET 200 may fabricate a reduced-void strained structure to enhance carrier mobility and the device performance.

It is understood that the FinFET 200 may undergo further CMOS processes to form various features such as contacts/vias, interconnect metal layers, dielectric layers, passivation layers, etc. It has been observed that the modified insulation and strained structure provides a given amount of strain into channel region of a FinFET, thereby enhancing the device performance.

One aspect of this description relates to a fin field effect transistor including a first insulation region and a second insulation region over a top surface of a substrate. The first insulation region includes tapered top surfaces, and the second insulation region includes tapered top surfaces. The fin field effect transistor further includes a fin extending above the top surface between the first insulation region and the second insulation region. The fin includes a first portion having a top surface below the tapered top surfaces of the first insulation region. The fin includes a second portion having a top surface above the tapered top surfaces of the first insulation region.

Another aspect of this description relates to a fin field effect transistor including a first insulation region over a top surface of a substrate, the first insulation region includes a tapered top surface. The fin field effect transistor further includes a second insulation region over the top surface of the substrate. The fin field effect transistor further includes a fin extending from the substrate, the fin located between the first insulation region and the second insulation region. The fin field effect transistor further includes a gate stack covering a first portion of the fin, the gate stack exposing a second portion of the fin, wherein a top surface of the first portion of the fin is above the tapered top surface, and a top surface of the second portion of the fin is below the tapered top surface.

Still another aspect of this description relates to a method of fabricating a fin field effect transistor (FinFET). The method includes forming a first insulation region and a second insulation region having respective top surfaces. The method further includes forming a fin between the first and second insulation regions, wherein the top surfaces of the first and second insulation regions are below a top surface of the fin. The method further includes forming a gate stack over a portion of the fin and over a portion of the first and second insulation regions. The method further includes tapering the top surfaces of the first and second insulation regions not covered by the gate stack.

While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

What is claimed is:
 1. A fin field effect transistor comprising: a first insulation region and a second insulation region over a top surface of a substrate, the first insulation region comprising tapered top surfaces, and the second insulation region comprising tapered top surfaces; and a fin extending above the top surface between the first insulation region and the second insulation region, wherein the fin comprises a first portion having a top surface below the tapered top surfaces of the first insulation region and the second insulation region, wherein the fin comprises a second portion having a top surface above the tapered top surfaces of the first insulation region and the second insulation region.
 2. The fin field effect transistor of claim 1, further comprising a gate stack over the second portion.
 3. The fin field effect transistor of claim 2, wherein the gate stack comprises: a gate electrode layer having a first side and a second side being opposite to the first side; and a pair of spacers, wherein a first spacer of the pair of spacers is on the first side of the gate electrode and a second spacer of the pair of spacers is on the second side of the gate electrode.
 4. The fin field effect transistor of claim 3, wherein sidewalls of the second portion are substantially coplanar with the pair of spacers.
 5. The fin field effect transistor of claim 3, wherein the gate electrode layer comprises at least one of Al, Cu, W, Ti, Ta, TiN, TiAl, TiAlN, TaN, NiSi, or CoSi.
 6. The fin field effect transistor of claim 3, wherein the first spacer or the second spacer comprises at least one of silicon oxide, silicon nitride, or silicon oxynitride.
 7. The fin field effect transistor of claim 1, wherein each tapered top surface of the tapered top surfaces comprises: a first surface parallel to the top surface of the substrate; and a second surface at an angle to the first surface.
 8. The fin field effect transistor of claim 7, wherein a ratio of a width of the first surface to a maximum width of each tapered top surface of the tapered top surfaces ranges from 0.05 to 0.95.
 9. The fin field effect transistor of claim 7, wherein a distance from the first surface to a top surface of the second portion ranges from 100 nanometers (nm) to 200 nm.
 10. The fin field effect transistor of claim 1, wherein each tapered top surface of the tapered top surfaces comprises a continuous curved surface.
 11. A fin field effect transistor comprising: a first insulation region over a top surface of a substrate, the first insulation region comprising a tapered top surface; a second insulation region over the top surface of the substrate; a fin extending from the substrate, the fin located between the first insulation region and the second insulation region; and a gate stack covering a first portion of the fin, the gate stack exposing a second portion of the fin, wherein a top surface of the first portion of the fin is above the tapered top surface, and a top surface of the second portion of the fin is below the tapered top surface.
 12. The fin field effect transistor of claim 11, wherein the gate stack comprises: a gate electrode layer having a first side and a second side being opposite to the first side; and a pair of spacers, wherein a first spacer of the pair of spacers is adjacent to the first side and a second spacer of the pair of spacers is adjacent to the second side.
 13. The fin field effect transistor of claim 12, wherein the gate electrode layer comprises at least one of Al, Cu, W, Ti, Ta, TiN, TiAl, TiAlN, TaN, NiSi, or CoSi.
 14. The fin field effect transistor of claim 12, wherein at least one of the first spacer or the second spacer comprises at least one of silicon oxide, silicon nitride, or silicon oxynitride.
 15. A fin field effect transistor comprising: a first insulation region and a second insulation region over a top surface of a substrate, each of the first insulation region and the second insulation region comprising at least one curved top surface; and a fin between the first insulation region and the second insulation region, wherein the fin comprises a first portion having an upper surface below the at least one curved top surface of the first insulation region and the at least one curved top surface of the second insulation region, wherein the fin comprises a second portion having a highest surface above the at least one curved top surface of the first insulation region and the at least one curved top surface of the second insulation region.
 16. The fin field effect transistor of claim 15, further comprising a pad layer over the highest surface of the fin.
 17. The fin field effect transistor of claim 5, further comprising a strained material over each of the first insulation region and the second insulation region.
 18. The fin field effect transistor of claim 15, further comprising a gate stack over the second portion.
 19. The fin field effect transistor of claim 18, wherein the gate stack comprises: a gate electrode layer having a first side and a second side being opposite to the first side; and a pair of spacers, wherein a first spacer of the pair of spacers is adjacent to the first side and a second spacer of the pair of spacers is adjacent to the second side.
 20. The fin field effect transistor of claim 19, wherein at least one of the first spacer or the second spacer comprises at least one of silicon oxide, silicon nitride, or silicon oxynitride. 